# std::is_class

From cppreference.com
< cpp‎ | types

Defined in header `<type_traits>` |  |
template< class T > struct is_class; |  | (since C++11)

`std::is_class` is a UnaryTypeTrait.

Checks whether `T` is a non-union class type. Provides the member constant value which is equal to true, if `T` is a class type (but not union). Otherwise, value is equal to false.

If the program adds specializations for `std::is_class` or `std::is_class_v`, the behavior is undefined.

### Template parameters

T | - | a type to check

### Helper variable template

template< class T > constexpr bool is_class_v = is_class<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is a non-union class type, false otherwise (public static member constant)

### Member functions

operator bool | converts the object to bool, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | bool
`type` | std::integral_constant<bool, value>

### Possible implementation

    namespace detail
    {
        template<class T>
        std::integral_constant<bool, !std::is_union<T>::value> test(int T::*);

        template<class>
        std::false_type test(...);
    }

    template<class T>
    struct is_class : decltype(detail::test<T>(nullptr)) {};

### Example

Run this code

    #include <type_traits>

    struct A {};
    static_assert(std::is_class<A>::value);

    class B {};
    static_assert(std::is_class_v<B>);
    static_assert(not std::is_class_v<B*>);
    static_assert(not std::is_class_v<B&>);
    static_assert(std::is_class_v<const B>);

    enum class E {};
    static_assert(not std::is_class<E>::value);

    union U { class UC {}; };
    static_assert(not std::is_class_v<U>);
    static_assert(std::is_class_v<U::UC>);

    static_assert(not std::is_class_v<int>);

    static_assert(std::is_class_v<struct S>, "incomplete class");
    static_assert(std::is_class_v<class C>, "incomplete class");

    int main() {}

### See also

is_union (C++11) | checks if a type is a union type (class template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/types/is_class&oldid=176478"